1. Technical Field of the Invention
The present invention relates to chassis systems for vehicles, more particularly to suspension devices for vehicles, and still more particularly to suspension devices which allow wheel camber control substantially independently of the vertical oscillations of the wheel.
2. The Related Art
Vehicular suspension devices have two main functions which must be fulfilled simultaneously at any moment during operation. One of these functions is that of suspending the vehicle, that is to say, permitting substantially vertical oscillations of each wheel in accordance with the load applied to the wheel. The other function of these devices is that of guiding the wheel, that is to say, controlling the angular position of the wheel plane.
The term xe2x80x9cwheel planexe2x80x9d refers to the plane, associated with the wheel, which is perpendicular to the axis of the wheel and which passes through the center of the contact area with the ground. The angular position of the wheel plane with respect to the body of the vehicle is defined by two angles, the camber angle and the steering angle. The camber angle of a wheel is the angle separating, in a transverse plane perpendicular to the ground, the wheel plane from the mid-plane of the vehicle. This angle is positive when the upper part of the wheel deviates from the mid-plane towards the outside of the vehicle, this being commonly termed xe2x80x9ccamberxe2x80x9d or xe2x80x9cpositive camberxe2x80x9d. Conversely, when this angle is negative, the term used is xe2x80x9ccounter-camberxe2x80x9d or xe2x80x9cnegative camberxe2x80x9d. The steering angle of a wheel is the angle separating, in a horizontal plane parallel to the ground, the wheel plane from the mid-plane of the vehicle.
On most vehicles, the camber angle (xe2x80x9ccamberxe2x80x9d or xe2x80x9ccamber anglexe2x80x9d will be used without distinction hereinbelow) is fixed for a particular position of the suspension and the steering; that is to say, theoretically it cannot vary independently of the suspension deflection or the steering. However, it undergoes variations induced by the deformations of the elements constituting the suspension device caused by the forces exerted on the wheel by the ground. These variations may be considerable. For example, an ordinary passenger car experiences camber variations of several degrees under the transverse forces developed on the tire on a curve, irrespective of the contribution of the roll of the vehicle body (which generally tilts in the same direction under the effect of centrifugal force). This xe2x80x9celasticxe2x80x9d variation of the camber causes the camber to increase (the camber tends towards positive values) for the outer wheel on the curve. Conversely, the camber decreases (it tends towards negative values) for the inner wheel on the curve. For a long time, these predictable variations have been incorporated into the design and adjustment compromises of the suspension devices for such ordinary vehicles in order to limit the harmful effects which they have on the functioning of the chassis system.
The camber has a great influence on the behavior of the vehicle and the performance of the chassis system. In particular, the performance of a tire is very variable depending on the configuration of its area of contact with the ground, and this configuration depends largely on the camber. The choice of the static camber angle is based mainly on these variations. Thus, for example, a large negative static camber is generally introduced on a racing vehicle in order to compensate for the variations due to the deformations of the tire under transverse force, as well as the suspension elements, even though they are much more rigid than on passenger cars, and due to the roll of the body. This configuration is both useful and acceptable in racing, since the criteria of grip on cornering are a major concern here. In contrast, on a passenger car, since the wear of the tires and the straight-line stability have more weight in the compromise being sought, a very slightly negative initial static camber is generally chosen. It is necessary to accept reduced slip thrusts, mainly on curves, when the effects of deformations of the tire and the elements of the ground contact system under the lateral forces on the positioning of the wheel plane are added to the effects of the roll of the vehicle.
In order to optimize the camber, in particular during transverse accelerations, suspension devices whose camber varies in accordance with the vertical deflection of the wheel have been designed. In this way, the roll experienced by the body of the vehicle can induce a useful variation of the camber which partly or totally compensates for the inclination of the body of the vehicle and the deformations described above. This is the case of the so-called xe2x80x9cmulti-linkxe2x80x9d systems. These devices require a specific design and vehicle architecture, which cannot be implemented on most current vehicles, for reasons of space requirements and cost. These systems react only to the consequence (deflection, rolling) of a transverse acceleration and not to the forces which cause it, thereby, on the one hand, delaying the effect of the correction. Moreover, to permit a sufficient variation of the camber, the kinematics of these systems require displacements of the position of the contact area with respect to the vehicle, called xe2x80x9ctrack changesxe2x80x9d, and these variations can also create difficulty. The range of camber corrections made possible by such systems is therefore relatively limited when the compromise necessary for correct functioning of the other load cases, such as travelling on a bumpy road, unilateral or in contrast simultaneous bouncing, is to be observed.
From the point of view of kinematics, in terms of degrees of freedom, suspension devices generally have only one degree of freedom (of the wheel or wheel carrier with respect to the vehicle). This degree of freedom permits vertical suspension movements which, as explained above, can be combined with limited camber variations.
Systems are known, however, in which the control of the camber is active; that is to say, the geometry modifications are controlled by movements of actuating cylinders, as described, for example, in the patent documents U.S. Pat. No. 4,515,390, U.S. Pat. No. 4,700,972 and DE 19717418. In these systems, at least a certain degree of additional freedom controlled by actuators has been permitted. These systems are very specific, since they cannot be used in most ordinary vehicles, in particular because of their space requirement, the considerable power necessary for the actuators and for costs reasons.
An object of the invention is to provide a suspension device of simple construction, which allows control of the camber substantially independently of the vertical oscillations of the suspension device and, more generally, of the movements of the body of the vehicle.
The foregoing and other objects of the invention are attained by a suspension device intended to connect a wheel carrier to a body of a vehicle, such device having means conferring to the wheel carrier, with respect to the body, a degree of camber freedom and a degree of suspension deflection freedom independent of one another. The means includes an intermediate support linking the wheel carrier to the body, the intermediate support being, on the one hand, articulated with respect to the body along a substantially vertical first axis and, on the other hand, articulated to the wheel carrier along a second axis, so that a rotation of the intermediate support about the first axis permits the degree of camber freedom. The suspension device of the invention has two degrees of freedom permitting independent suspension and camber movements. The camber movement is effected in a simple manner by virtue of the rotation of the intermediate support about a substantially vertical first axis. xe2x80x9cSubstantially verticalxe2x80x9d means in this context that the axis of rotation can be inclined, for example up to an inclination of 30xc2x0 from vertical as will be seen from the detailed description of the drawings.
Preferably, with the wheel carrier being intended to carry a wheel of radius xe2x80x98rxe2x80x99 and the wheel being intended to rest on the ground by way of its contact area, the suspension device is configured so that the camber movement of the wheel carrier with respect to the body allows, about a mean position, a first instantaneous center of rotation situated in a range of from 2.5 r above the ground to r below the ground, and preferably in a range from r above the ground to r below the ground. The fact that the camber movement is effected about an instantaneous center of rotation situated at a limited distance from the contact area makes it possible to limit the track changes during cambering or counter-cambering and also to limit the supply of energy necessary in the case of active control of the camber.
In a preferred embodiment, the first instantaneous center of rotation is situated in a range of from 0.2 r above the ground to 0.4 r below the ground.
In order to ensure stable functioning, the suspension device is preferably configured so that it is close to the equilibrium in the aforementioned mean position in the absence of transverse force exerted by the ground on the wheel in the contact area and also preferably configured so that, in the absence of camber variations, the transverse force exerted by the ground on the wheel in the contact area generated in the course of the suspension deflection does not exceed a reasonable limit compared to the weight of the vehicle. These conditions are preferably achieved by virtue of a suspension device in which the camber movement of the wheel carrier with respect to the first axis allows, about the mean position, a second instantaneous center of rotation situated substantially in the plane of the wheel. More preferably, the position, relatively to the wheel center, of the second instantaneous center of rotation forms an angle of less than 15xc2x0, preferably less than 5xc2x0, with the wheel plane.
To permit passive functioning, the first instantaneous center of rotation can preferably be situated below the plane of the ground, so that transverse forces exerted by the ground on the wheel in the contact area induce an inclination of the wheel carrier with respect to the body towards decreasing camber, when the transverse forces are directed towards the inside of the vehicle, and towards increasing camber, when the said transverse forces are directed towards the outside of the vehicle. In this passive functioning case, linked to the transverse forces, the suspension device can have means for measuring the angular displacement of the intermediate support to deduce the transverse forces therefrom.
The transverse forces which act on the wheel in the contact area originate mainly from the transverse acceleration acting on the vehicle travelling on a curve. The same acceleration applies substantially in the same way on each element of the vehicle, in particular on all the elements making up the chassis system. In particular, the wheel and its associated parts tend to lean towards the outside of the curve. This obviously has to be reckoned with when designing the device. However, this effect can also be profited from in the context of the present invention if it is made useful for the camber variation. This is possible if, for example, the mass of the intermediate support is positioned in relation to its axis of rotation in such a way that the centrifugal force which acts on its center of mass generates a torque which tends to rotate the intermediate support in the desired direction, that is to say, towards the desired camber change for such a transverse acceleration. In that case, the effect of transverse acceleration is used (at least partially) to help achieve the desired camber change. Hence, this effect can allow the provision of a suspension device in accordance with the invention, in which the first instantaneous center of rotation is closer to the ground level but which passive camber behavior is still satisfactory.
Preferably, the intermediate support is linked to the wheel carrier so that the steering is substantially independent of the camber and the suspension device can further have means for controlling the steering. Steering is generally considered independent of camber if a given camber variation (for example 5xc2x0) induces a steering variation of less than 10% of the given camber variation (for example 0.4xc2x0).
In certain conditions, it may be necessary or beneficial further to provide control means capable of influencing the camber of the wheel. These means can comprise an elastically deformable element opposing the camber movement, the deformable element being comprised of, for example, elastomeric articulations.
The invention also relates to a device intended to connect, in addition, an opposite wheel carrier belonging to the same axle as the wheel carrier.
Finally, the invention relates to a vehicle equipped with such a suspension device.
Several embodiments of the invention will be described in order to illustrate the features and explain the principles thereof. Naturally, other embodiments of the invention are possible, as suggested by the numerous variants.